CELL-DIVISION AND DEVELOPMENT. 



139 



vi 



FIG. 8. 



pole of the ovum four nuclei (vi) are seen with only a very 

 slight amount of protoplasm sur- 

 rounding them, so that when stained 

 with haematoxylin they stand out 

 very clearly on the white yolk; these 

 nuclei have resulted from the division 

 of the nucleus D of the stage repre- 

 sented in Fig. 7, and when followed 

 in later stage will be found to sink 

 into the yolk and become the vitello- 

 phags of the embryo, still later, as 

 will be shown in a paper shortly to 

 appear, giving rise to certain mesodermal structures. Sur- 

 rounding the area occupied by these vitellophags, is a circle of 

 twelve nuclei (me) whose surrounding protoplasm stains very 

 deeply, being thus in marked contrast to the vitellophags ; 

 these cells and their descendants later concentrate on the ven- 

 tral surface of the embryo, and give rise to a mass of cells 

 from which the endoderm and so much of the mesoderm as is 

 not represented by the vitellophags will be formed. Finally, 

 scattered over the rest of the ovum are sixteen nuclei (ec), 

 whose protoplasm, though distinctly visible, does not stain so 

 deeply as that of the mesendodermal nuclei, and which give 



rise later to the ectoderm of the 

 embryo. 



In the second place it will be 

 seen that a cleavage of the yolk 

 has supervened, the entire surface 

 of the egg being divided into hex- 

 agonal or pentagonal areas, in the 

 center of each one of which is one 

 of the nuclei. This yolk cleavage 

 is, however, superficial, as can be 

 seen from Fig. 9, which represents 

 a section of an ovum in the stage 

 under consideration. From this 

 section it will be seen that the nuclei and their surrounding 

 protoplasm have now reached the surface of the yolk; in fact, 



FIG. 9. 



